The present invention relates generally to the preparation of solutions of polyaniline and, more particularly, to the preparation of concentrated solutions (15%-40% w/w) having molecular weight averages (Mw)xe2x89xa7120,000 and number averages (Mn)xe2x89xa730,000 in the pernigraniline, emeraldine, and leucoemeraldine base forms of polyaniline using certain primary and secondary amines as gel inhibitors, and the preparation of solutions ( greater than 20% w/w) having molecular weight averages (Mw) less than 120,000 and number averages (Mn) less than 30,000 in these forms of polyaniline using certain primary, secondary and tertiary amines as gel inhibitors, in polar aprotic organic solvents, which solutions may be processed into films, coatings, and fibers that are highly electrically conducting after subsequent exposure to acid.
Dopable xcfx80-conjugated polymers (CPs) possess alternating double and single bonds along the polymer main chain repeat units, such as those found in the family of polymers known as polyaniline, show potential for a variety of commercial applications such as chemical separations, electromagnetic interference shielding, protection of metals from corrosive environments, antistatic coatings, and current carrying fibers. Polyaniline is a commercially attractive polymer since, unlike many other dopable xcfx80-conjugated polymers, it is both environmentally stable and can be made electrically conducting by acid treatment.
Electrical conductivity ("sgr") of CPs is possible due to electron mobility along (intrachain) and between (interchain) polymer chains in a solid state article. The magnitude of the conductivity depends upon the number of charge carriers (n) which is determined by the extent of doping with oxidizing or reducing chemical agents (or in the special case of polyaniline, with an acid), the charge on these carriers (q), and on the combined interchain and intrachain mobilities (xcexc). These relationships are related by: "sgr"=nqxcexc. In order to obtain high conductivities, n is usually maximized by a chemical doping process (generation of electrons or holes on the polymer chain), so that conductivity becomes dependent on the mobility of the carriers. At the maximum doping levels, it is the mobility of the charge carriers which must be increased to obtain higher conductivity. Mobility of charge carriers in some cases depend upon the polymer""s morphology once it is xe2x80x9cfrozenxe2x80x9d into a non equilibrium glassy solid state article determined by processing conditions. Interchain mobility depends upon the statistical distribution of conformational features such as bond and torsion angles, interchain distances, packing density, orientation, fractional crystallinity, free volume, etc. By contrast, intrachain mobility depends upon the degree and extent of xcfx80-conjugation and defects along the polymer chains, and the polymer chain conformations. It is therefore desirable to develop improved processing procedures which allow control over the factors governing mobility in order to generate higher conductivities in polyaniline.
Optimally doped CPs contain approximately equal numbers of carriers, but exhibit order of magnitude differences in conductivity depending on sample preparation methods. For instance, stretch-aligned optimally doped transpolyacetylene films exhibit conductivities of the order of 105 S/cm, while identically doped nonstretched films are in the range of 103 S/cm. Structural features which favor enhanced carrier mobility, e.g., chain alignment through mechanical stretching, are important for obtaining high transport coefficients and 3-dimensional metallic transport. Interchain mobility depends upon the statistical distribution of conformational features such as bond and torsion angles, interchain distances, packing density, orientation, fractional crystallinity, free volume, and the spatial distribution and ordering of dopants. By contrast, intrachain mobility depends upon the degree and extent of xcfx80-conjugation, molecular weight, number of defects along the polymer chains, and the polymer chain conformations themselves. Processing procedures which exert control over the structural factors which govern mobility must be utilized in order to achieve metallic-like conductivities in solid-state xcfx80-conjugated polymers.
However, despite efforts to develop viable, processing routes for polyaniline (PANI), the proccessing barriers intrinsic to this material have not yet been overcome: (a) production of practical, high-quality fibers with adequate strength; and, simultaneously (b) achievement of metallic state conductivity predicted by theory. Melt extrusion is not feasible since this polymer, like most conducting polymers (CPs), decomposes before melting. Solution processing of PANI into film, fiber, or coatings is extraordinarily difficult due to: (a) extremely poor solubility in solvents; (b) rapid polymer gelation times at low ( greater than 5% w/w) total solids content; and, (c) strong aggregation tendency due to interchain attractive forces, e.g., hydrogen bonding. Furthermore, these problems prevent utilization of high molecular weight polyaniline at concentrations exceeding 15% w/w which are generally required to produce strong fibers by dry-jet wet spinning techniques or impact resistant coatings, or films by conventional rolling techniques.
There are three oxidation states for polyaniline (PANI): a) the fully oxidized form known as pernigraniline base; b) the intermediate form called emeraldine base; and c) the fully reduced form which is given the name leucoemeraldine base. The general formula describing each of these three primary oxidation states for PANI is: [(C6H4xe2x80x94NHxe2x80x94C6H4xe2x80x94NHxe2x80x94)1xe2x88x92x][(C6H4xe2x80x94N=C6H4=Nxe2x80x94)x], where x ranges from 0 to 1. When x=1, the polymer is in the fully oxidized (pernigraniline) form and each nitrogen of the polymer repeat unit is a tertiary amine, i.e., all are imine nitrogens. When x=0, the polymer is in the fully reduced (leucoemeraldine) oxidation state and every nitrogen of the polymer repeat unit is a secondary amine. However, when x=0.5, the polymer is in an intermediate (emeraldine) oxidation state with equal numbers of amine and imine nitrogens in the polymer repeat unit. These structures were deduced by Green and Woodhead early in this century.
Emeraldine base is the well-known form of PANI and this xe2x80x9cA-Bxe2x80x9d base polymer exhibits the structure: 
where the repeat unit is fully planar and has one quinoid (Q) and three benzenoid (B) ring monomers (tetrameric repeating units each containing two secondary amine and two tertiary imine nitrogen atoms).
The untreated EB is itself an electrical insulator. When powders of EB are treated with acid solutions, the imine nitrogen atoms extract protons from solution with the acid counterion associating with the polymer chain to maintain overall charge neutrality. When less than 50% of the available imine nitrogens are coordinated to form quaternary iminium salt complexes; that is, immersion in solutions having pH values between 2 and 7, the polymer becomes a semiconductor and is called a bipolaron (see FIG. 1b hereof), since charge carriers delocalized along the xcfx80-conjugated polymer backbone are spinless. Immersion in more concentrated acid solutions (pH less than 2) generates polarons (see FIG. 1c hereof) since, due to self-localized reorganization of electronic states, the mobile charge carriers are now sufficiently delocalized to produce mobile spins. Thus, treatment of EB (which has a conductivity of less than 10xe2x88x9210 Siemen/cm [S/cm]) with an excess of concentrated acid solution (pH less than 1) results in an electrically conductive polymer having a conductivity of about 1 S/cm. Under these latter doping conditions, the maximum number of charge carriers (n) have been generated on the polymer since all of the nitrogen atoms, available as protonation sites, are occupied. Thus, the conductivity of EB can be increased by over ten orders of magnitude ( less than 10xe2x88x9210 to 1 S/cm) by varying the number of protonated imine sites (carriers) through exposure to an equilibrium pH concentration of acid (H+Axe2x88x92), forming thereby a quartenary emeraldine iminium salt (ES). The average dopant concentration is given by the molar ratio of anions (Axe2x88x92) to nitrogen atoms (N) as y=Axe2x88x92/N, where y has values up to 0.5 (100% doping). Although this acid doping process involves no net charge transfer, it profoundly alters the local bond order of the primary chain and, simultaneously, the ring torsion of the labile phenylene units. In general, doping is occurs when a conducting polymer is exposed to a controlled amount of an oxidizing (p-type) or reducing (n-type) agent. In the special case of EB, a Lewis or a Brxc3x6nsted acid is used to induce charge carriers onto the polymer in order to improve conductivity.
In xe2x80x9cConcurrent Reduction And Modification Of Polyaniline Emeraldine Base With Pyrrolidine And Other Nucleophiles,xe2x80x9d by Chien-Chung Han and Rong-Chyuan Jeng, Chem. Commun. 1997, pages 553-554, it is shown that polyaniline emeraldine base can easily be modified by pyrrolidine and other nucleophiles such as piperidine, morpholine, alkane-1-thiols, and mercaptoacetic acid through concurrent reduction and substitution mechanisms. Such modifications may significantly reduce the conductivity of the resulting polymers.
Lewis acid (A)-base (B) interactions lead to the formation of acid-base complexes (A:B). A Lewis acid is an electron pair acceptor (EPA), while a Lewis base is an electron pair donor (EPD). An A:B complex is formed when there is orbital overlap between a filled electron orbital of high energy from the Lewis base and an empty low energy orbital from the Lewis Acid. Hydrogen bonding is a Lewis acid-base type interaction in which the EPA is also a Brxc3x6nsted acid; that is, a proton attached to a heteroatom. The hydrogen bond is a secondary bond formed to another atom by a covalently bonded hydrogen atom. Schematically, this interaction is described by:
Rxe2x80x94Xxe2x80x94H+:Yxe2x80x94R less than = greater than Rxe2x80x94Xxe2x80x94H...Yxe2x95x90R
(EPA) (EPD) (H-Bonded Adduct)
where X and Y atoms both have electronegativities greater than hydrogen; for example, C, N, P, O, S, and halogens. Imine nitrogens (tertiary amines) are good proton acceptors, while primary and secondary amines can be either proton donors or acceptors.
The commonly reported polyaniline synthesis describes the heterogeneous radical chain polymerization of aniline at 0xc2x0 C. in 1 N aqueous HCl, and leads to the acid salt form of polyaniline (See, e.g., A. G. MacDiarmid et.al., xe2x80x9cConducting Polymersxe2x80x9d, Alcacer, L., ed., Riedel Pub., 1986, p.105, FIG. 1c). When this polyaniline salt powder is immersed in an excess of a strong aqueous base, it is deprotonated to yield EB (See FIG. 1a hereof. Most polyaniline investigations have employed materials having molecular weights with weight average (Mw) less than 120,000 and number average (Mn) less than 30,000 which are produced by these synthetic conditions (See, e.g., E. J. Oh et al., xe2x80x9cPolyaniline: Dependency Of Selected Properties On Molecular Weight,xe2x80x9d Synthetic Metals, 55-57, 977 (1993).
In U.S. Pat. No. 5,312,686 for xe2x80x9cProcessable, High Molecular Weight Polyaniline And Fibers Made Therefrom,xe2x80x9d which issued to Alan G. MacDiarmid et al. on May 17, 1994, a procedure for preparing high molecular weight polyaniline is reported. The method involves reducing the standard reaction temperature to xe2x88x9230xc2x0 C., by adding 5 M LiCl to the reaction mixture, thereby producing high-molecular-weight EB. The molecular weight of the resulting polymer may be varied from (Mw)=250,000 to greater than (Mw)=400,000 by controlling the rate at which the initiator is added to the cold reaction mixture, and the reaction temperature. These high molecular-weight polyanilines exhibit poor solubility and have short gelation times. A complex cycling procedure of acid doping, followed by undoping with aqueous base reportedly led to improved solubility and concentrated solutions in N-methyl-2-pyrrolidinone (NMP). Unfortunately these solutions were discovered to rapidly gel when prepared in the 1-3% w/w range in NMP. Thus, there exists a need for developing procedures to process high molecular weight polyaniline.
The utility of polyaniline EB with (Mw)xe2x89xa7120,000 and (Mn)xe2x89xa730,000 has been limited. However, in order to process high-quality fibers possessing good mechanical properties, it is known in the art that solution concentrations of a particular polymer should be in the 15-30% (w/w) range. Moreover, it is desirable to use the highest molecular weight polymers that will dissolve in solvents in the target concentration range. Tensile strength and modulus, flex life, and impact strength all increase with increasing molecular weight. Typically, molecular weights (Mw)xe2x89xa7120,000 and (Mn) greater than 30,000 are preferred. Such solutions are suitable for dry-wet or wet-wet fiber spinning processes that produce high quality fibers, and also for the generation of films, coatings and other useful objects.
The emeraldine base form of polyaniline is reported to be soluble in NMP at the 1-5% weight level. Such solutions may be cast into dry dense films after the wet film is thermally treated to remove the solvent. Films prepared in this manner, when immersed in a concentrated acid solution, have a conductivity of between 1 and 5 S/cm. Few other organic solvents for EB, such as N,N,Nxe2x80x2Nxe2x80x2-tetramethyl urea and N,Nxe2x80x2-dimethyl propylene urea (DMPU) as examples, have been reported in the literature. All of these solvents have amide functional groups, which tend to form strong hydrogen bonds between the carbonyl group of the solvent and the secondary amine groups of the emeraldine base, thus encouraging limited solubility at dilute concentrations prepared from low molecular weight polymer. However, solubilities of even low molecular weight EB (0xc2x0 C. synthesis, (Mw) less than 120,000, (Mn) less than 30,000) in such solvents is poor ( less than 1-5% w/w). Solutions prepared from NMP above this concentration range exhibit rapid gelation. (see, e.g., E. J. Oh et al., supra). Oh et al. observed that the gelation time is both inversely proportional to the weight percent of EB in NMP and to it""s molecular weight. S. A. Chen et al. in xe2x80x9cConductivity Relaxation Of 1-Methyl-2-Pyrrolidinone-Plasticized Polyaniline Filmxe2x80x9d, Macromolecules 28, 7645 (1995), have reported evidence for a strong hydrogen bond interaction of the Cxe2x95x90O group from NMP with the secondary amine (NH) functional groups of EB. Presumably, it is the imine nitrogens from the polymer which are strongly attracted to hydrogen atoms of the secondary amines on adjacent chains. This strong attractive force promotes interchain hydrogen bonding which serve as physical cross-links between chains and leads to rapid gelation in EB solutions, or in the solid state article (FIG. 2a).
Emeraldine base solutions can be processed into free-standing films. If such films are stretched over a hot pin before immersion in a concentrated acid solution, and then subsequently treated with an acid, conductivities of as great as 200 S/cm may be obtained. A. G. MacDiarmid et al xe2x80x9cTowards Optimization of Electrical and Mechanical Properties of Polyaniline: Is Cross-Linking Between Chains the Key?xe2x80x9d, Synthetic Metals, 55-57, (1993) 753, shows that stretch alignment of emeraldine base films [prepared from dilute (1-3% w/w) EB in N-methyl-2-pyrolidinone (NMP) solutions], over a hot pin at 120xc2x0 C. to a 2-5xc3x97 draw ratio, increases the films fractional crystallinity (from xcx9c5 to 50%) and additionally increases the anisotropic conductivity of the maximally acid doped film from 1 to 200 S/cm, in the direction parallel to the stretch. Hence, this example demonstrates the importance of manipulating the parameters which control carrier mobility (xcexc) in the solid state articles to enhance physical properties such as conductivity.
Preparation of EB solutions having  greater than 10% w/w from DMPU has been reported (See e.g., K. T. Tzou, R. V. Gregory xe2x80x9cImproved Solution Stability And Spinnability Of Concentrated Polyaniline Solution Using N,N-DimethylPropylene Urea As The Spin Bath Solventxe2x80x9d Synthetic Metals 69, 109-112, 1995). Here also, a synthetic procedure which yields low molecular weight EB ((Mw) less than 120,000, (Mn) less than 30,000) was reported. The solutions were stable long enough for the authors to spin a fiber which exhibited high conductivity; however, the details of processing, and the solubility limits, are lacking, and the resulting mechanical properties of the fiber would be much improved if higher molecular weights were accessible in their solvent systems.
A second category of reported solvents for polyaniline includes acids, such as m-cresol, formic acid, methanesulfonic acid, sulfuric acid, as examples. Solubility derives from the basic nature of the EB polymer which forms ionic coordination complexes between the acid and the imine nitrogens of the polymer. Solubility increases as the strength of the acid increases ( greater than 10% w/w for sulfuric acid, 1-5% w/w in m-cresol and formic acid). It is doubtful that EB is truly dissolved in such acid solutions; but rather, it is more likely that the solutions consist of a fine dispersion of polyaniline particles. Processing EB in such solutions is not desirable since 1. The solvents are hazardous; 2. Strong acids can either over-oxidize emeraldine or chemically substitute on the polymer rings; and 3. The resulting polymers tend to degrade if stored in solution for more than a few days. Additionally, even though partially soluble in acid media, EB fibers spun from acid solution have been found to be mechanically weak.
A major obstacle to the fabrication of commercially useful articles, such as high quality fibers, hollow fibers, or articles having other useful geometries, from solutions of polyaniline, therefore, is the poor solubility of the polymer in solvents suitable for processing using conventional polymer engineering methods. Such solutions exhibit a strong tendency to form gels on a relatively short time scales due to interchain hydrogen bond formation, even for dilute solutions. The instability is such that the solutions cannot be extruded through spinnerette orifices because they gel too rapidly or form particulate material which clogs the spinnerette tip, causing unsafe pressure increases in the spin line which represent a significant health risk situation to operators.
U.S. Pat. No. 5,135,682 for xe2x80x9cStable Solutions Of Polyaniline And Shaped Articles Therefrom, which issued to Jeffrey D. Cohen and Raymond F. Tietz on Aug. 4, 1992 discloses a procedure for preparing stable dry-wet spinning solutions of EB in the 10-30% w/w range. Stable, spinnable solutions were prepared using 1,4-diaminocyclohexane, 1,5-diazabicyclo (4.3.0) non-5-ene, or by dissolving EB in NMP with the addition of specified quantities of cosolvents consisting of either pyrrolidine (Py) [11% EB; 33% Py; and 56% NMP w/w/w] or ammonia. The amount of pyrrolidine added as cosolvent, compared to the amount of the EB added to NMP solution, can be expressed as the ratio of moles Py/moles EB tetrameric repeat unit, which in their preferred embodiment is 15.5. (The molecular weight of the EB repeat unit is 362 g/mol, and that of Py is 71.13 g/mol). Poor quality fibers were observed for the NMP/Py solutions (see e.g. ibid. Example 5). The work was further described in xe2x80x9cPolyaniline Spinning Solutions and Fibers,xe2x80x9d by C.-H. Hsu, J. D. Cohen and R. F. Tietz in Synthetic Metals 59, 37 (1993), where the authors suggested that the physical degradation of the polyaniline fibers, especially after exposure to an acid, was likely due to the addition of Py or ammonia cosolvents, as a result of chemical interactions between the cosolvent and the polymer. Molecular weights reported from the described synthetic procedure were approximately (Mn)=20,000 and (Mw)=120,000. Synthetic conditions were carried out at xe2x88x928xc2x0 C. without LiCl added to the reaction mixture.
In U.S. Pat. No. 5,147,913 for xe2x80x9cCross-Linked Polymers Derived From Polyaniline And Gels Comprising The Same,xe2x80x9d which issued to Alan G. MacDiarmid and Xun Tang on Sep. 15, 1992, the preparation of cross-linked polymers of polyaniline by providing a substantially linear polymer which comprises polyaniline and/or a polyaniline derivative, mixing the linear polymer with a liquid in which the cross-linked polymer is substantially insoluble, and cross-linking the polymer through agitation, is described. Preferred liquids for preparing such gels include NMP. A preferred embodiment for forming such gels is utilization of EB in NMP at concentrations  greater than 5% w/w.
In xe2x80x9cStabilization of Polyaniline Solutionsxe2x80x9d by Debra A. Wrobleski and Brian C. Benicewicz, Polymer Preprints 35, 267 (1994) and in xe2x80x9cStabilization of Polyaniline Solutions Through Additives,xe2x80x9d U.S. Pat. No. 5,583,169 which issued to Debra A. Wrobleski et al. on Dec. 10, 1996, the authors report the addition of hindered amine antioxidants and UV absorbers to up to 5% w/w solutions of EB in NMP to increase the gelation time for such solutions. Although molecular weights for the EB are not reported, the described synthesis must have produced EB with weight average molecular weights below (Mw) less than 100,000 and number averages (Mn) less than 30,000.
Accordingly, it is an object of the present invention to provide a method for dissolving high concentrations (between 15 and 40% w/w) of polyanilines [weight averages (Mw)xe2x89xa7120,000 and number averages (Mn)xe2x89xa730,000] without significant gel formation over a time period sufficient to process the solution obtained thereby into articles.
Another object of the invention is to provide a method for preparing solutions having high concentrations (between 15 and 40% w/w) of polyanilines [weight averages (Mw)xe2x89xa7120,000 and number averages (Mn)xe2x89xa730,000] from which articles can be prepared having improved electrical conductivities and mechanical properties.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for preparing solutions having between 15% and 40% by weight of (Mw)xe2x89xa7130,000, (Mn)xe2x89xa730,000 (high-molecular weight) emeraldine base form of polyaniline hereof includes the steps of: mixing a solvent for polyaniline with a primary amine, a secondary amine or a mixture of a primary amine and a secondary amine such that the molar ratio of the amines to polyaniline tetramer repeat unit is between 0.1 and 5.0, forming thereby a solution; and dissolving polyaniline having (Mw)xe2x89xa7130,000 and (Mn)xe2x89xa730,000 in the solution thus prepared, whereby a solution is formed which is stable over a chosen time period.
In another aspect of the invention, in accordance with its objects and purposes, as embodied and broadly described herein, the method for preparing solutions having  greater than 20% by weight of (Mw) less than 120,000, (Mn) less than 30,000 (low-molecular weight) emeraldine base form of polyaniline hereof includes the steps of: mixing a solvent for polyaniline with a primary amine, a secondary or a tertiary amine or mixtures thereof such that the molar ratio of the amine to polyaniline tetramer repeat unit is between 0.1 and 5.0, forming thereby a solution; and dissolving polyaniline having (Mw) less than 120,000, (Mn) less than 30,000 in the solution thus prepared, whereby a solution is formed which is stable over a chosen time period.
In yet another embodiment of the invention, in accordance with its objects and purposes, as embodied and broadly described herein, the method for preparing solutions having  greater than 20% by weight of (Mw) less than 120,000, (Mn) less than 30,000 emeraldine base form of polyaniline hereof includes the step of dissolving a chosen amount of polyaniline having (Mw) less than 120,000, (Mn) less than 30,000 in a bifunctional solvent therefor having both an amide group and an amine group the bifunctional solvent, forming thereby a solution which is stable over a chosen time period.